1. Field of the Invention
The present invention relates generally to the field of antennas. More specifically, the present invention discloses a dish antenna with a ring focus suitable for use in satellite broadcasting.
2. Statement of the Problem
Parabolic reflector antennas are widely used in the field of satellite television broadcasting. With the improvements in receiving/transmitting equipment used on the satellites, more powerful beams are transmitted to the ground and that in turn allows the use of smaller antennas than those used before. Axis-symmetrical dual reflector antennas, especially ring focus antennas, occupy less volume and are preferable for use in mobile applications, such as on recreational vehicles, automobiles, small boats, or in portable antenna systems. At the same time, smaller reflector antennas usually have decreased efficiency relative to larger ones due to a variety of reasons, including spillover and the comparably large shadow from the feed or sub-reflector.
The prior art in this field includes a number of axially-symmetric ring focus antennas. For example, U.S. Pat. No. 3,162,858 (Cutler) discloses an axially-symmetric ring focus antenna that has low noise and high gain. Its feed has a radial transmission line that evenly distributes energy in the aperture and provides lower spillover. However, its feed has a complicated tuning mechanism that would be difficult to manufacture (e.g., by casting) in mass production due to the shape of deep annular corrugations 40 on the surfaces of the transmission line. Also, this antenna requires the size of the reflector to be at least ten times the diameter of the antenna feed. Furthermore, having corrugations 40 on both surfaces of the radial transmission line delivers a cosine-shaped peak intensity of the signal to the primary reflector. As a result, the periphery of the reflector (having the largest surface area) is not illuminated evenly with the middle section of the reflector, which results in lower aperture efficiency of the antenna.
U.S. Pat. No. 6,724,349 (Baird et al.) shows another example of an axis-symmetrical antenna with a feed that would be relatively simple to manufacture, has low spillover and a low level of polarization. This antenna includes a multimode circular waveguide horn with a series of different size cross-sections to improve the radiation patterns of the horn itself. But, the feed is not made to be used in ring focus antennas and radiation from the horn is partially blocked by the sub-reflector, which would reduce the efficiency of the antenna.
Prata et al., “Displaced-Axis-Ellipse Reflector Antenna for Spacecraft Communications” disclose another axis symmetrical antenna of very high efficiency (90%) with a small size sub-reflector (3.3 wavelength). But, the size of the main reflector is about 30 wavelengths and is therefore too large to be used for mobile antennas. The primary reflector is deep enough to allow the phase center of the horn to be aligned with the focal point of the ellipse used to shape the sub-reflector but again, that results in increased size of the antenna.
U.S. Pat. No. 7,408,522 (Ahn et al.) shows another antenna of greatly reduced size implementing an axis-symmetrical antenna of the same general type (axial displaced ellipse, or ADE) as Prata et al. This antenna has aperture efficiency at the level of 60-65%, which is lower than Prata et al. due to the size of the sub-reflector and feed being larger relative to the size of the main reflector and its inability to illuminate the surface of the main reflector evenly to achieve higher aperture efficiency due to parameters chosen for the design of the feed parts.
Thus, there remains a need for an antenna to address these shortcomings in the prior art, and in particular a small high-efficiency antenna that can be easily manufactured.